DESCRIPTION

The fopen() function opens the file whose name is the string pointed to
by filename and associates a stream with it.
The argument mode points to a string beginning with one of the following
sequences (Additional characters may follow these sequences.):
``r'' Open text file for reading. The stream is positioned at the
beginning of the file.
``r+'' Open for reading and writing. The stream is positioned at the
beginning of the file.
``w'' Truncate to zero length or create text file for writing. The
stream is positioned at the beginning of the file.
``w+'' Open for reading and writing. The file is created if it does not
exist, otherwise it is truncated. The stream is positioned at
the beginning of the file.
``a'' Open for writing. The file is created if it does not exist. The
stream is positioned at the end of the file. Subsequent writes
to the file will always end up at the then current end of file,
irrespective of any intervening fseek(3) or similar.
``a+'' Open for reading and writing. The file is created if it does not
exist. The stream is positioned at the end of the file. Subse-
quent writes to the file will always end up at the then current
end of file, irrespective of any intervening fseek(3) or similar.
The mode string can also include the letter ``b'' either as last charac-
ter or as a character between the characters in any of the two-character
strings described above. This is strictly for compatibility with ISO/IEC
9899:1990 (``ISO C90'') and has no effect; the ``b'' is ignored.
Finally, as an extension to the standards (and thus may not be portable),
mode string may end with the letter ``x'', which insists on creating a
new file when used with ``w'' or ``a''. If path exists, then an error is
returned (this is the equivalent of specifying O_EXCL with open(2)).
Any created files will have mode "S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP |
S_IROTH | S_IWOTH" (0666), as modified by the process' umask value (see
umask(2)).
Reads and writes may be intermixed on read/write streams in any order,
and do not require an intermediate seek as in previous versions of stdio.
This is not portable to other systems, however; ANSI C requires that a
file positioning function intervene between output and input, unless an
input operation encounters end-of-file.
The fdopen() function associates a stream with the existing file descrip-
tor, fildes. The mode of the stream must be compatible with the mode of
the file descriptor. When the stream is closed via fclose(3), fildes is
closed also.
The freopen() function opens the file whose name is the string pointed to
by filename and associates the stream pointed to by stream with it. The
original stream (if it exists) is closed. The mode argument is used just
as in the fopen() function.
If the filename argument is NULL, freopen() attempts to re-open the file
associated with stream with a new mode. The new mode must be compatible
with the mode that the stream was originally opened with:
o Streams originally opened with mode ``r'' can only be reopened
with that same mode.
o Streams originally opened with mode ``a'' can be reopened with
the same mode, or mode ``w''.
o Streams originally opened with mode ``w'' can be reopened with
the same mode, or mode ``a''.
o Streams originally opened with mode ``r+'', ``w+'', or ``a+''
can be reopened with any mode.
The primary use of the freopen() function is to change the file associ-
ated with a standard text stream (stderr, stdin, or stdout).

RETURN VALUES

Upon successful completion fopen(), fdopen(), and freopen() return a FILE
pointer. Otherwise, NULL is returned and the global variable errno is
set to indicate the error.

ERRORS

[EINVAL] The mode argument to fopen(), fdopen(), or freopen()
was invalid.
The fopen(), fdopen() and freopen() functions may also fail and set errno
for any of the errors specified for the routine malloc(3).
The fopen() function may also fail and set errno for any of the errors
specified for the routine open(2).
The fdopen() function may also fail and set errno for any of the errors
specified for the routine fcntl(2).
The freopen() function may also fail and set errno for any of the errors
specified for the routines open(2), fclose(3) and fflush(3).